Insulation For Exterior Walls: What’s the Best Type to Use?

There are many different types of insulation we can use when we build or renovate homes. But manufacturers tell us that their products are the best, and installers recommend the types that they prefer to use. So, it can be mind-boggling for consumers who don’t have any knowledge or insight into the hows and whys of insulation.

If you’ve read our ultimate guide to home insulation you’ll know that not only is insulation available in different forms but it is also manufactured for different applications. This means that the insulation you use in your roof, floor spaces, and in the walls of your house will need to suit these spaces.

I talked about R-values in my post about types of energy-efficient wall construction and explained what R-values are all about. In essence, it indicates the ability of insulation material to reduce heat flow. The higher the R-value, the more heat flow is reduced.

In this post, I’m going to focus on the types of insulation you can use to insulate exterior walls. I  aim to help you decide which is the best type to use when you build or renovate.

You will see that the R-value of insulation, the type of construction used for building, and your location are all key to knowing which type of insulation to choose. You will also see that there are various rules and regulations that will guide you. 

How Location Affects Your Exterior Wall Insulation Needs

Insulation in your exterior walls lowers heating and cooling costs and also improves comfort. But it stands to reason that you’re not going to need the same R-values in Florida as you do in Montana or New York.

The solution, devised by the Department of Energy (DOE), comes in the form of eight U.S. climate zones that are sub-categorized by moist, dry, and marine. You will see from the map that these cut across states. For example, here in Florida, it’s much hotter in the south, which is classified Zone 1, while the rest of the state is Zone 2.

So, even though building codes in the various U.S. states specify minimum R-value requirements for insulation, these need to be flexible due to microclimates. Generally, the best approach is to opt for wall insulation with a higher R-value than the minimum specified. 

The International Code Council (ICC) has an International Energy Conservation Code (IECC) that is the national model standard for energy-efficient residential construction that is recognized by federal law. Insulation R-values are specified for ceilings, floors, and both wood-frame and mass walls, which include concrete, concrete block, insulated concrete forms, masonry cavity, and brick walls. These vary according to the eight climatic zones, as you can see from the chart  below.

Because the IECC may differ from energy codes that various states adopt, it is essential to check local building codes as well – just to be sure. The North American Insulation Manufacturers Association (NAIMA) Insulation Institute has useful links that show how the different U.S. states have incorporated IECC R-value requirements in their local codes.

IECC Insulation Requirements for Walls
Climate Zone	Wood Frame Walls R-value	Mass Walls R-value
1	13	3/4
2	13	4/6
3	20 or 13+5h	8/13
4 except Marine	20 or 13+5h	8/13
5 and Marine 4	20 or 13+5h	13/17
6	20+5 or 13+10h	15/20
7 & 8	20+5 or 13+10h	19/21

Source: NAIMA

In the table above, all R-values are minimums. If insulation is installed in an exterior cavity wall that is less than the design thickness of the insulation used, the installed R-value of the insulation mustn’t be less than the R-value specified.

Where two figures are used, like 4/6, R-4 means continuous insulation on the interior or exterior or R-6 cavity insulation at the interior of a basement wall. Where you have a plus sign, eg 13+5, R-13 means cavity insulation + R-5 continuous insulation.

As the DOE points out, the specifications for insulation for 2 x 4 inch and 2 x 6 inch stud-framed walls also vary. They recommend R-13-R-15 for 2 x 4 and R-19-R-21 for 2 x 6 without relating the specs to climate zones.

The Insulation R-Value Rule

R-values are a vital factor in terms of the efficacy of insulation, which is why the Federal Trade Commission (FTC) created the R-Value Rule. Simple in concept, it requires manufacturers, sellers, and professional installers of insulation products to disclose the R-value.

Because there is a vital need to keep consumers properly informed, there are standard ASTM tests to establish accurate R-values. The rule also requires information on fact sheets to cover the importance of air sealing and proper insulation installation.

Types of Exterior Wall Insulation and R-values

So now all you have to do is match the R-value recommended for your climate with an insulation type that is suitable for exterior walls.

Well, it’s not quite that simple, because, as the DOE points out, R-values are dependent on correct installation, and while some types, like blankets and batts, are reasonably easy for DIYers to install, most require professional installation. The heating and/or cooling system in your home will also impact the type of insulation you should choose.

You can use many different insulation products to achieve the required R-values in exterior walls. But don’t forget to check whether it complies with your local climate needs and your local code.

Typical types include:

Insulation Blankets & Pre-Cut Batts (panels)

Insulation blankets and batts made of fiberglass are probably the most common type used in the U.S. They are typically R-13 or R-30 if 3.5 or 12 inches thick and can be used for all new walls, including foundation walls. Other materials manufacturers use include mineral rock or slag wool, and both plastic and natural fibers.

You can buy batts and blanket rolls in different widths that are suitable for the standard spacing of 2 x 4 inch (R-11 for low-density, R-13 for medium-density, and R-15 for high-density) and 2 x 6 inch (R19 and R21) wall studs. Some products are available with facings including kraft paper, vinyl, or foil that provide an additional air or vapor barrier. Batts with flame-resistant facing are ideal for exterior basement walls.

The DOE has a detailed chart for fiberglass batts but advises consumers to check actual thickness with manufacturers because there will often be discrepancies. They are all standard medium-density specs unless specified as being high Performance (high-density).

Fiberglass Batt Insulation for Walls
Thickness (Inches)	R-Value
3½	12 – 16
3⅝	15 – 20
3½ (high-density)	34 – 40
6 – 6¼	27 – 34
5¼	33 – 39
8 – 8½	37 – 45
8 (high-density)	45 – 49
9½	39 – 43
12	55 – 60

Source: Department of Energy

Fitted between the studs of an exterior wall, blankets and batts are relatively inexpensive and can be installed by do-it-yourselfers.

We’ve also done a pretty good (we think) comparison between fiberglass and spray foam here.

Loose-Fill & Blown-In Insulation

Loose-fill and blown-in types of insulation include fiberglass, cellulose, and mineral slag or rock wool. It conforms to any size or shape space without disturbing the structure or finishes and is great for filling irregularly shaped areas, especially those around obstructions like pipework in exterior walls. 

• Fiberglass used for loose-fill insulation usually contains between 40% and 60% recycled glass content.
• Rock wool is a man-made material that contains natural minerals like diabase or basalt.
• Slag wool is also man-made, from the blast-furnace slag that forms on the surface of molten metal.
• Cellulose insulation is made from recycled paper protects, mainly newsprint. It has a very high recycled material content, usually as much as 82%-85%. Manufacturers add the mineral borate for resistance to fire and insect infestation.

Generally blown into place with special equipment, loose-fill insulation is sometimes poured into exterior wall cavities, especially where they are difficult to reach and in renovations. Typically R-values are R-30 or R-50 if 8 or 23 inches thick, but it does depend on the product.

Recommended Specifications for Loose-Fill Materials
	Fiberglass	Cellulose	Rock Wool
Density in lb/ft3 (kg/m3)	0.5-1.0 (10-14)	1.5-2.0 (24-36)	1.7 (27)
Weight at R-38 in lb/ft2 (kg/m2)	0.5-1.2 (3-6)	1.25-2.0 (6-10)	1.6-1.8 (8-9)

Source: Department of Energy

When insulating exterior attic walls, you can use all three materials for ½ inch drywall 16-inch on center and ⅝ inch drywall 24-inch on center. Cellulose and rock wool aren’t suitable for ½ inch drywall 24-inch on center.

We’ve also done a blown-in cellulose vs spray foam, along with a cellulose vs. Rockwool post here!

Foamed-In-Place & Sprayed Foam

Foam that is sprayed or foamed-in-place is made from various material types including cementitious, phenolic, polyisocyanurate, and polyurethane. According to the DOE, some types of liquid foam insulation materials have an even higher R-value than traditional batt insulation of the same thickness.

There are two basic types of foamed-in-place polyurethane products, open-cell, and high-density closed-cell. Open-cell polyurethane spray foam is typically R-12.6 if 1-inch thick, while closed-cell polyurethane spray foam is usually R-6.5 if 1-inch thick. Open-cell products are lighter and less expensive, but you shouldn’t use them for below-ground foundation walls because they can absorb water.

Happily, most of the foam materials we use today don’t use harmful chlorofluorocarbons (CFCs) or hydrochlorofluorocarbons (HCFCs).

Rigid Insulation for Walls

Rigid insulation generally reduces energy losses throughout the entire frame of houses, but there are many options and the challenge is to know which one to choose. A professional installer will be able to help you.

Here are some basic guidelines for the different types of foam board. The most common types are:

• Expanded polystyrene foam board has R-values ranging from R-3.8-4.4 if 1-inch thick
• Extruded polystyrene foam board has R-values ranging from R-5 if 1-inch thick
• Polyisocyanurate (polyiso) foam board has R-values ranging from R-6.5 if 1-inch thick

Foam board can be used on the outside of new exterior walls and the inside walls of existing exterior walls when the house is built with concrete blocks. Insulating cores increase the R-value of walls, but they need highly specialized skills for installation. When adding insulation to existing block walls, this must be done from the inside.

Insulating Concrete Forms

Foam boards or blocks manufactured as insulated concrete forms (ICFs) that are poured in situ, can also be used to insulate new exterior walls. They are also suitable for foundation walls underground. Installed as part of the building structure, they are known to create a high thermal resistance of around R-20.

Structural Insulated Panels

Structural insulated panels (SIPs) are available as foam board or a liquid foam insulation core that is suitable for new exterior walls. According to the DOE, SIP-built houses are superior to those built using traditional construction methods and they can be built more quickly. When properly installed, SIPs can result in energy savings of 12%-14%.

SIPs are manufactured with various insulating materials, most commonly polystyrene or polyiso foam. Fire safety is a concern that is overcome if gypsum board is used to protect the facing and foam. Also, because well-built SIP structures are so airtight, it is essential for buildings to have controlled fresh-air ventilation that will help to prevent any possible moisture problems inside.

Reflective Insulation Systems & Radiant Barriers

Radiant barriers and reflective insulation work by reflecting radiant heat.

Reflective insulation is effective in reducing the downward flow of heat. It is easy to fit between the wood-frame studs of exterior walls during construction, which makes it suitable for DIY use.

Radiant barriers are particularly useful in hot, sunny climates and work well to keep the interior of homes cool.

Final Wrap Up

The best type of insulation to use for your exterior walls will keep your home warm in winter and cool in summer.

Building new homes provides us with a myriad of opportunities for installing good quality, effective insulation in exterior walls. You’ll be spoiled for choice. It’s more difficult when renovating houses, but can be done. The choice of insulation material and installation method for renovations will be more limited and will depend largely on the wall construction method used.

Whether you are renovating or insulating newly-built exterior walls, it is vital to use insulation that has R-values that are suitable for your climate. It must also work with the construction methods and materials used for the building and the type of heating and cooling system you install. Be guided by the various local and international codes, and read available fact sheets for more information.